1. Motion, Forces, and Simple Machines
Chapter 19

2. Section 1
Motion

3. Motion Think about the motion of a skateboard on the half pipe
Are you getting faster or slower as you go down the pipe?
Are you getting faster or slower as you go up the pipe?
What happens when you get to the high point of your jump?
We can describe motion in one of two ways
What is the distance we have traveled
How long did it take to travel that distance?

4. Speed
Bike riders can speed up or slow down several times during a time period
One way to find how fast the bike rider is going is to calculate average speed - the distance traveled over a certain period of time
To calculate average speed (s) you divide the total distance traveled (d) by the time it took you travel that distance (t) s=d/t.

5. Speed If an airplane travels 1,350km in 3 hours, what is its speed?
S = d/t
S = 1,350 ÷ 3
S = 450km/h

6. Speed
Average speed is helpful if you don’t care about the details of the motion
A car that travels 640km in 8h has traveled 80km/h
During that time, the car could have been stuck in traffic, stopping at lights, or driving at high speed on the highway.
It may be important to know how fast you are going at any given time, you may be traveling an average of 90km/h but going 100km/h at one point.
This is important if the speed limit is 90km/h!
To keep from speeding, a driver needs to know instantaneous speed – the speed of an object at any given time.

7. Speed
Sometimes when driving, the driver may turn on the cruise control which makes the car go the same speed while it is on.
In this case, the instantaneous speed is not changing.
When the instantaneous speed does not change, this is called constant speed
If an object is moving at a constant speed, we can calculate how far it has gone (d) by multiplying the average speed (s) by the time it traveled that speed (t) or d = s · t.

8. Speed
How far can a marathon runner go if she runs at 16km/h for 24 minutes or 0.4h?
D = speed x time
D = 16km/h · 0.4h
D = 6.4km

9. Velocity
The velocity of an object is the speed of an object and its direction of motion
The velocity of an object will change when EITHER speed or direction change
If you are walking down the street at a constant speed and turn the corner, your speed has not changed but your direction of travel has changed; therefore, your velocity has changed.
If you slow down, but continue in the same direction, your velocity has changed.
If you slow down to make a turn, your velocity has changed.

10. Acceleration
If you are on a skateboard at the top of the halfpipe, your speed is zero, you are at rest
As you start down, you begin to speed up as you go down the pipe. If the pipe were steeper, you would speed up faster.
How could you describe how your speed is changing?
Acceleration describes how velocity changes with time. Acceleration (a) is the change in velocity (final speed – initial speed) divided by the time needed for that change to occur (t)

11. Acceleration
Suppose you are sliding down a waterslide at a speed of 8m/s. There is a drop and you increase your speed to 18m/s in 5 seconds. Find your acceleration.
A = (final speed – initial speed) / t
A= (18m/s - 8m/s)/5s
A = 10m/s / 5s
A= 2m/s2

12. Graphing Speed
Picture yourself skating down a hill, across a level surface, then back up the hill.
If you were to graph your speed, it would look like the chart to the right
As you go down hill, your acceleration is in the same direction as your motion so your speed increases as seen by the upward slanting (blue) line.
As you go across the flat area, you speed stays constant and there is no acceleration, as seen by the straight (green) line
As you go uphill, your speed decreases when your acceleration is in the opposite direction of your motion as seen by the downward slanting (red) line.

13. Newton’s 3 Laws of Motion
Section 2
Newton’s 3 Laws of Motion

14. Force What causes an object to move? (discuss with your partner)
Any time you push or pull an object, you are exerting a force on it
A force is a push or a pull
Force is measured in newtons, which is about the amount of force needed to lift a quarter pound hamburger

15. Force and acceleration
Exerting a force causes its motion to change, causing the object to accelerate
When you throw a ball, your hand exerts a force on the ball causing it to speed up
Force can also change the direction of an objects motion
After you throw the ball, if no one catches it, it will curve downward to the ground
Gravity pulls the ball downward causing it to change direction
Force can cause an object to change its speed, direction, or both

16. Balanced and Unbalanced forces
More than one force can act on an object without causing its motion to change
If you and a friend push on a door in opposite directions with the same force, the forces are balanced and the door will not move
If you and a friend push on the door and one friend pushes harder than the other, the forces are unbalanced and the door will move in the opposite direction of the greater force
If more than one force acts on an object, the forces combine
The combination of all forces on an object is called net force

17. Gravity
Gravity is a force that pulls objects downward towards the Earth
If you hold a basketball and let it go, gravity will pull it to the ground
Objects do not have to be touching to exert a gravitational pull, but the farther away the objects are, the weaker the gravitational force will be between them
The gravitational force is also weaker between objects of less mass, you have less gravitational pull on your book than the Earth does!

18. Newton’s First Law of Motion
A scientist named Sir Isaac Newton discovered many things about motion and used these discoveries to create 3 laws of motion
His first law is the law of inertia.
If you push a book across the table, it will eventually stop, the same with the ball you threw, it will eventually roll to a stop.
You may think this means for an object to stay in motion, you must exert a net force on it at all times
However, Newton and others figured out that an object could be moving if no net force is acting upon it

19. Newton’s First Law of Motion
Newton’s first law states that an object will not change its motion unless a force is exerted on it.
This means the ball or the book with continue moving until some other force stops its motion
The book slid across the table and stopped because of friction – a force that resists movement between two surfaces that are in contact.
The size of the friction force depends on the two surfaces involved, the rougher the surfaces, the greater the friction force

20. Newton’s First Law of Motion
You may have also noticed that the heavier the object, the more force is required to move it
Even if the object has wheels, such as a car, it is still hard to move the object
This is because of inertia – the tendency to resist a change in motion
The more mass an object has, the greater its inertia is

21. Newton’s Second Law of Motion
Newton’s second law states that a net force acting upon an object changes the motion of the object
When a net force acts upon an object, its acceleration depends upon its mass
The more mass an object has, the less acceleration it will receive
If the force required to move a grocery cart is exerted on a refrigerator, the refrigerator will not accelerate as much as the grocery cart
In other words, the more mass, the less acceleration

22. Newton’s Third Law of Motion
Newton’s third law states that when one object exerts a force on another object, the second object exerts and equal force back on the first object, or for every action, there is an equal and opposite reaction
When a swimmer pushes against a wall, the wall pushes back and the swimmer changes direction
If you were to jump off a small boat, you would exert a force on the boat causing it to move backwards, the boat would also exert a force on you causing you to move forward
You have more mass than the boat, so the boat more than you do, if the boat had more mass, you would move more than the boat

23. Section 3
Work and Simple Machines

24. Bell Work Read pages 570-571 and answer the following questions
What is work?
Are you doing work if you push against a wall? Why or why not?
Are you doing work if you push a computer cart across the room? Why or why not?
Are you doing work if you carry a book across the room? Why or why not?

25. Work
If you lift a box off the ground, have you done work? Discuss with your partner.
If you then walk with that box, are you doing work? Discuss with your partner.
You may think of work as household chores or having a job at an office, factory, or farm, but in science the definition of work is a little more specific
Work is done when a force causes an object to move in the same direction the force is applied

26. Effort doesn’t equal Work
For work to be done, two things must happen
First, you must apply a force and second, you must move an object in the same direction the force is applied
When you lift the box, the box moves in the same direction of the force and work is done
However, when you carry the box, your arms are lifting upward while the box is moving forward
In this case no work is being done

27. Calculating Work
To do work, a force must be applied, the greater the force applied that make the object move, the more work is done
Lift a pile of books form the floor to your waist takes more work than lifting a shoe the same distance
Even though both objects move the same distance, it requires more force to lift the books
To calculate work, you use the formula Work = force x distance or W = F x d

28. Calculating Work
Force is measured in Newtons (N) and distance is measured in meters (m)
Work and energy are both measured in joules (J) named after the British physicist, James Prescott Joules, that discovered that work and energy are related
Example: A weight lifter lifts a 500-N weight a distance of 2m from the floor to a position above his head. How much work does he do?
W = F x d
W = 500 N x 2 m
W = 1000 J

29. Bell work

30. What is a Machine
How many machines have you used today? Discuss with your partner
A machine is a device that makes work easier
The can opener in Figure 16 transfers the small force applied by your hand into a larger force that cuts of the lid
The can opener is a simple machine
Simple machines – are machines that use only one movement

31. What is a Machine
All you do is turn the handle and the machine does the work
Simple machines include inclined plane, wedge, screw, lever, wheel and axle, and the pulley
Compound machines – a combination of simple machines
Machines can either change the size of the force applied or change the direction of the force
The number of times the force is multiplied by a machine is called the mechanical advantage

32. The Pulley
A pulley is a machine with a groove, like a wheel, that has a rope or chain running through the groove
A pulley changes the direction on the input force
A rope thrown over a rail can serve as a pulley
In this case, the pulley ONLY changes the direction of the force, so its mechanical advantage is 1 (You must supply 100N of force to lift a 100N object)

33. The Pulley
If you link two pulleys together, each pulley does half the work meaning you only have to supple half the work to lift the same object. (You supply 50N to lift a 100N object)

34. The Lever This was probably the first machine invented by humans
The lever is a rod or plank that pivots on a fixed point
Levers can increase force or the distance over which force is applied

35. The Lever There are three types, or classes, of levers
A first class lever has the fulcrum, or pivot point, between the input force and the output force (a screwdriver opening a can of paint)
A second class lever has the output force between the fulcrum and input force (a wheelbarrow)
A third class lever has the input force located between the fulcrum and output force (a hockey stick)
1st class
2nd class
3rd class

36. The Wheel and Axle
A wheel and axle is made of two round objects that spin on the same axis
Usually the larger round object is the wheel and the smaller is the axle
Doorknobs and car tires are examples of a wheel and axle

37. The inclined Plane
The inclined plane is sloped surface, often called a ramp
The inclined plane increases the distance over which work is done
Imagine trying to lift a couch 1m off the ground
Lifting the couch straight up would require lots of work
By sliding it up a ramp, you increase the distance the couch moves and in turn decrease the amount of force required to move the couch so less work is done

38. The Wedge
A wedge is a moving inclined plane with one or two sloping sides
Your front teeth are wedges
Wedges change the direction of the input force from a downward force to a side to side (cutting) force
Knives and axes are also wedges used for cutting

39. The Screw The screw is an inclined plane wrapped around a post
The screw changes the direction of the input force
When you drive a screw into wood, the turning force pulls the screw deeper into the wood and friction between the threads (inclined plane) and wood hold the screw in place.